The present invention relates to fiber optic networks, and more particularly to chromatic dispersion compensation in fiber optic networks.
Fiber optic networks are becoming increasingly popular for data transmission due to their high speed, high capacity capabilities. FIG. 1 illustrates a simplified optical network 100. A fiber optic network 100 could comprise a main loop 150, which connects primary locations, such as San Francisco and New York. In between the primary locations are local loops 110, 120, which connect with the main loop 150 at connector points 140 and 160. A local loop could be, for example, an optical system servicing a particular area. Thus, if local loop 110 is Sacramento, an optical signal would travel from San Francisco, add and drop channels with Sacramento""s signal at connector point 140, and the new signal would travel forward to connector point 160 where channels are added and dropped with local loop 120, and eventually to New York. Within loop i 10, optical signals would be transmitted to various locations within its loop, servicing the Sacramento area. Local receivers 170 would reside at various points within the loop to convert the optical signals into signals in the appropriate protocol format. Loops 110 and 120 may also exchange channels directly with each other through a connector point 130 between them.
A common and well-known problem in the transmission of optical signals is chromatic dispersion of the optical signal. Chromatic dispersion refers to the effect where the channels within a signal travel through an optic fiber at different speeds, i.e.; longer wavelengths travel faster than shorter wavelengths. This is a particular problem becomes more acute for data transmission speeds higher than 2.5 gigabits per second. The resulting pulses of the signal will be stretched, possibly overlap, and make it more difficult for a receiver to distinguish where one pulse begins and another ends. This seriously compromises the integrity of the signal.
A conventional solution to this problem is the use of fixed dispersion compensators at various locations in the network as needed. These devices compensate for a fixed dispersion value by canceling the dispersion in the fiber link. The difficulty with using fixed dispersion compensators is that an optical link or network is rarely uniform. Different systems in the network may use different types of fiber, as well as different types of receivers with different tolerances. The fibers within a system may be of different lengths necessitated by landscapes, building locations, etc. Also, different systems may contain devices from different vendors, each with its own dispersion tolerance. Thus, in order to obtain as close to optimum dispersion compensation through the entire system, the dispersion must be manually determined for every fiber and optical in the system, and a dispersion compensator with the appropriate fixed value must be purchased and installed. This solution is costly to the network operator in both money and time. Many hours of human labor must be expended to measure the dispersion of each fiber in the system and to order, inventory, install, and setup the fixed dispersion compensators. To do the job properly at extremely high bit rates, a network operator must remove transmission traffic from the fiber link, measure the dispersion in the fiber link, and then manually insert the fixed dispersion compensator. Many operators xe2x80x9cguessxe2x80x9d at the dispersion based upon the length of the fiber and statistics of dispersion. They then order a fixed dispersion compensator, which approximates the dispersion. For example, assume the residual dispersion using conventional dispersion compensation at the end of a fiber transmitting a standard NRZ pulse format has a value of 1200 ps/nm. The xe2x80x9cguessxe2x80x9d method will work for a pulse of 2.4 Gb/s, will be difficult to achieve at 10 Gb/s, and will not work at 40 Gb/s. Thus, the operator must xe2x80x9cguessxe2x80x9d within the dispersion tolerance. In general, if the residual dispersion of the transmission link is less than the dispersion tolerance of the Transceiver, the system will operate properly. At extremely high bit rates such as 40 Gb/s, meeting this condition will be extremely rare.
Accordingly, there exists a need for a method and system for automatically compensating for chromatic dispersion in an optical network, which does not require the manual determination, installation and purchase of a dispersion compensation of a fixed value. The method will save network operators both money and time. The present invention addresses such a need.
The present invention provides a method and system for compensating for chromatic dispersion in an optical network. The method and system includes analyzing an optical signal at a first location in the optical network and determining if an eye of the optical signal has a desired shape at the first location. The method and system further includes automatically adjusting a dispersion of the optical signal at a second or at any location in the optical network if the optical signal does not have the desired shape; providing feedback concerning the shape of the adjusted optical signal at the first location; and automatically readjusting the dispersion of the optical signal at the second location based upon the feedback until the optical signal has the desired shape at the first location. The method and system of the present invention automatically compensates for dispersion in an optical system. This saves the network operator considerable time and raises the reliability. The method and system could be used to provide a continuous range of dispersion compensation to particular locations in the network or to the network as a whole, and to store information in a database to be used to produce dispersion maps for the network. It also provides the ability to dial in dispersion for changing fiber characteristics and pre-tuning in a switching of wavelengths from one system to another. Finally, it avoids extraordinary expense by allowing the approximation of the installed fiber dispersion without requiring field measurements.